1. Field of the Invention
Embodiments of the present invention relate generally to gas sorption testing, and particularly to a thin-film sample holder for gas sorption measurements.
2. Description of the Related Art
In the field of nanotechnology, it is understood that a given material can behave in a significantly different manner when arranged differently on the nanoscale, that is, on the level of individual atoms or clusters of atoms. For example, the chemistry of a given material can be altered by inducing a particular mechano-chemical strain in the material on the nanoscale during formation. In this way, with an appropriate nanoscale configuration, ordinarily inert materials have been shown to posses catalytic properties, and seemingly constant thermodynamic properties of a material, such as the enthalpy of formation of a metal with hydrogen, can be altered and even optimized for a particular application.
Thin-film deposition processes are well-suited for forming large numbers of materials that each can be organized differently on the nanoscale, such as nanotubes, etc. or in very thin layers with nanometer dimensions. Accurate measurement of the gas sorption properties of thin films, i.e., absorption, adsorption, desorption, chemisorption and physisorption, is problematic, however, since sorption testing apparatus known in the art are designed for sorption testing of bulk powders rather than thin films. When measuring the sorption properties of a bulk powder, a relatively large quantity of gas sorbing sample material is used relative to the free-gas volume of the test gas present in the sample chamber. In this way, a measurable pressure change in the sample chamber takes place during sorption testing, even at elevated pressures. For example, the PCTPro-2000, available from Hy-Energy LLC, Newark, Calif., is configured to perform sorption testing on a material sample with a sample chamber having a free-gas volume of approximately 0.5 ml after the placement of a material sample having a mass of approximately 10 to 1000 milligrams in the chamber. Relative to such bulk material samples, the mass of a thin-film material deposited on a substrate that can be tested in a conventional sorption tester sample chamber can be smaller, having, for example, up to one or more orders of magnitude less mass than a typical bulk sample. In addition, because of the geometry of a thin-film on a substrate, when placed in a conventional sorption tester sample chamber the substrate generally occupies a very small portion of the chamber volume, leaving a high free-gas volume. Thus, because the ratio of sample chamber free-gas volume to sample material mass is so high when testing a thin-film sample on a substrate, the pressure drop produced by gas sorption of the thin film is not accurately measurable using conventional pressure measuring devices.
To test a greater amount of thin-film sample material, the thin film can be removed from underlying substrates and tested as a bulk material. Such an approach allows larger masses of material to be tested while reducing the free-gas volume in the sample chamber. But because the process of mechanically removing a thin film from a substrate is likely to significantly alter the nanoscale properties of the thin-film sample material, and therefore the gas sorption behavior of the sample material, in-situ testing of a thin film as deposited on a substrate (the film and substrate together being referred to herein as a thin-film substrate) is a more rigorous and reliable approach.
Alternatively, a larger mass of thin-film sample material can be sorption tested by configuring a sample chamber to contain an entire full-sized thin-film substrate, such as a 6 inch diameter silicon wafer. FIG. 1 illustrates a wafer-sized sample chamber 100 configured for attachment to a conventional sorption testing apparatus. Wafer-sized sample chamber 100 is a “clamshell” design, configured to contain an entire substrate 103. Substrate 103 is a standard thin-film substrate, such as a 6 inch or 8 inch silicon wafer. Substrate 103 has a thin film 107 deposited thereon, where thin film 107 includes a gas-sorbing material to be tested in wafer-sized sample chamber 100. Substrate 103 is positioned on a substrate support (not shown) in wafer-sized sample chamber 100 between lid 101 and base 102, and test port 110 is fluidly coupled to a sorption-testing apparatus using a leak-resistant means known in the art. A clamping mechanism (not shown) exerts closing force 106 so that lid 101 and base 102 press against sealing member 105 with sufficient force to allow pressurization of wafer-sized sample chamber 100 during sorption testing of thin film 107.
Prior to sorption testing, material samples are typically isolated from atmospheric moisture and other contaminants by being handled in a controlled environment, such as an argon-purged glove box or other isolation chamber. Because wafer-sized chamber 100 is configured for testing a full-sized substrate (i.e., substrate 103), and because wafer-sized chamber 100 has a simple two-piece clamshell configuration, the design of wafer-sized chamber 100 facilitates the loading of a test substrate therein while contained in a glove box. The use of substrate 103 also allows for a greater mass of sample material to be tested than can be deposited on a substrate small enough for use in a conventional sorption testing chamber.
However, wafer-sized chamber 100 is not suited for performing sorption testing since such tests are commonly performed at high pressures, e.g., tens to hundred's of atmospheres. First, closing force 106 needed when wafer-sized chamber 100 is pressurized to 100 atmospheres or more is prohibitively large, requiring an impracticably large and bulky apparatus. Second, the free-gas region 104 of wafer-sized chamber 100 is too large to allow accurate sorption measurements. Although a higher mass of thin film 107 can be sorption tested in wafer-sized chamber 100 than in a standard-sized sorption sample chamber, the ratio of free-gas volume to sample material mass is still too high for an accurately measurable pressure drop to take place during most sorption tests—particularly higher pressure tests. Lastly, the potential for leakage from free-gas region 104 past sealing member 105 is too high for reliable sorption measurements. This is because wafer-sized chamber 100 has a relatively large sealing surface, i.e., sealing member 105, and any leakage out of wafer-sized chamber 100 during a sorption test directly affects the accuracy of the test. Further, the leakage rate across sealing member 105 increases as the pressure inside wafer-sized chamber 100 increase, and decreases the more that a compression force is exerted on sealing member 105. The compression force is equal to the amount by which closing force 106 exceeds the minimum force necessary to hold lid 101 and base 102 together. Thus, at higher pressure sorption tests, there is more impetus for leakage across sealing member 105 at the same time that the compression force on sealing member 105 is reduced.
Accordingly, there is a need in the art for a sorption sample chamber that can accurately perform gas sorption measurements on thin-film samples at high pressures, and facilitates loading and unloading of thin-film samples while contained in a glove box or other isolation chamber.